1. Field of the Invention
The present invention relates to plasma processes, and in particular to optical systems and methods for detecting and analyzing optical emissions from the plasma along the length of the plasma.
2. Discussion of the Background
Plasma is used in various types of industrial processes, such as semiconductor manufacturing (e.g., integrated circuit fabrication and printed wiring board interconnections), medical equipment manufacturing and automobile manufacturing. One common use of plasma is for etching away materials in an isolated or controlled environment. One or more plasma compositions may etch various types of materials, including glasses, silicon or other substrate materials, organics such as photoresist, waxes, plastics, rubbers, biological agents, and vegetable matter, and metals such as copper, aluminum, titanium, tungsten, and gold. Plasma is also utilized for depositing materials, such as organics and metals, onto an appropriate surface by various techniques, such as via chemical vapor deposition. Sputtering operations may also utilize a plasma to generate ions that sputter away material from a source (e.g., metals, organics) and deposit those materials onto a target. Surface modification operations also use plasmas, such as surface cleaning, surface activation, surface passivation, surface roughening, surface smoothing, micromachining, hardening, and patterning.
Creating a plasma for manufacturing processes is typically done by introducing a low-pressure process gas into a plasma duct surrounding a work piece (substrate), such as an integrated circuit (IC) wafer. The molecules of the low-pressure gas in the chamber are ionized by a radio frequency (RF) power source to form a plasma that flows over the substrate. The plasma duct is used to maintain the low pressure required for the plasma and to serve as a structure for attaching one or more electrodes that couple electrical power to the plasma.
Plasma may be created from a low-pressure process gas by inducing an electron flow that ionizes individual gas molecules by transferring kinetic energy through individual electron-gas molecule collisions. Typically, electrons are accelerated in an electric field such as one produced by RF power. This RF power may have a low frequency (below 550 KHz), high frequency (13.56 MHz), or a microwave frequency (2.45 GHz).
Etching may be performed by plasma etching or reactive ion etching (RIE). A plasma etching system may include a single RF power source, or a plurality of such sources operating at one or more frequencies with a corresponding number of electrodes, at least one of which is located within the process chamber. A plasma is generated adjacent the substrate, the latter typically being co-planar with the electrode and supported by a substrate support member within the process chamber. The RF energy may be coupled to the plasma by capacitive means, by inductive means, or by both capacitive and inductive means. The chemical species in the plasma are determined by the source gas(es) used.
A plasma may also be used in chemical vapor deposition (CVD) to form thin films of metals, semiconductors or insulators (or, conducting, semiconducting or insulating materials) on a semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply the required reaction energy for depositing the desired materials. Typically, RF energy is used to produce this plasma.
Unfortunately, it is difficult to quickly assess the quality of the etching or deposition process in plasma processing. Presently, one of the main methods used to assess processing quality for a given plasma process involves the steps of processing a wafer in the reactor, removing the wafer, and then examining the wafer. (Those steps may be tedious and costly.) Furthermore, changes in the process due to equipment malfunctions, such as defects in mass flow controllers, almost always reduce process yields, and cannot generally be corrected until after test wafers have been processed and examined, which is a timely and expensive proposition.
Optical emission spectroscopy is a method currently used to detect a process endpoint in plasma etching systems. U.S. Pat. Nos. 5,658,423, 6,090,302 and 5,347,460 disclose different optical emission spectroscopy methods. Optical emission spectroscopy is performed in situ and is possible because the plasma excites certain atomic and molecular species present in the plasma and causes them to emit light of wavelengths that are characteristic of the species present in the etch chemistry. Plasma properties, including ion/electron density, electron temperature and relative chemical species concentrations, can be deduced from collecting and analyzing the optical emissions from the plasma.
In an optical monitoring system for performing optical emission spectroscopy, specific wavelengths of the light emitted from the plasma are selected and fed to detectors, such as photodiodes, photomultipliers, and array detectors, which convert the light intensities into electrical signals. It is known that the intensity of the detected raw signals is related to the concentration of excited species. By selecting wavelengths that correlate to the reaction products of the particular process, the process may be monitored either at specific wavelengths or at all wavelengths by a spectral scan.
In a particular application, end-point detection of a plasma process of a substrate having multiple layers may be performed by selecting a wavelength corresponding to the emissions generated by a layer below the layer being etched. When the layer being etched has been completely removed from the underlying layer, the chemical composition of both the gas phase and the remaining layer changes. Product species from the etched layer are no longer generated, and the concentration of some reactants increases because they are no longer being consumed by the reaction. These chemical changes show up as changes in optical emission intensities of the plasma.
Recognizing that plasma properties can vary over the length of the plasma, particularly along the axial direction of the duct in which the plasma is formed, the present invention relates to plasma processes, including optical systems and methods for detecting and analyzing optical emissions from the plasma along the length of the plasma.
According to a first aspect of the present invention, an apparatus detects and analyzes the spectra of light emitted by a plasma at multiple locations along the plasma as contained in a plasma duct. One such plasma duct has an outer wall with a plurality of locations (either in the same or different windows) transparent to the optical emissions. Such an apparatus may include a plurality of optical systems (e.g., adjacent the one or more windows), each capable of redirecting light entering therein. A beam splitter is arranged relative to the optical systems so as to receive light from the optical systems and direct the light along first and second optical paths. A first optical filter having a first bandwidth is arranged in the first optical path, and a second optical filter having a second bandwidth is arranged in the second optical path. First and second detectors are respectively arranged downstream of the first and second optical filters so as to detect light passing through the respective filters and convert the light into respective first and second electrical signals. Those electrical signals are representative of the intensity of light incident the first and second detectors.
In an alternate embodiment, instead of having a plurality of optical systems, a single optical system is arranged so as to be (1) translatable along the plasma duct and (2) in optical communication with the plasma through plural locations in the plasma duct (either in the same or different windows). The optical system may include an optical fiber that couples a collection optical system (xe2x80x9ccollection opticsxe2x80x9d) arranged adjacent the plasma duct and a receiving optical system (xe2x80x9creceiving opticsxe2x80x9d) at the end of the optical fiber opposite the collection optics.
According a second aspect of the present invention, the spectral properties of light emitted by a plasma at different regions of the plasma are measured. One possible environment is the apparatus of the first aspect, although the method can be used in any apparatus that supports measurements at multiple locations. The method sequentially collects light generated by the plasma in different areas. The sequentially collected light is separated into at least two paths and the sequentially collected light in each path is passed through a respective optical filter. Each optical filter has a different spectral bandwidth than the other filters so that each component of the spectrum can be isolated and measured. The filtered light is then detected from each path using corresponding detectors. Each detector can then generate a corresponding electrical signal representative of the intensity of the light detected.
In one embodiment of that method, the step of sequentially collecting light is performed by providing at least one optical system adjacent the plasma duct and shuttering all but one optical system at a time so that the optical emissions from different regions of the plasma can be analyzed.
In another embodiment of that method, the plasma properties are adjusted by selectively activating one or more electrodes based on the information contained in the electrical signals from the detectors.